Electrophotographic developing process

ABSTRACT

IN AN ELECTROPHOTOGRAPHIC LIQUID DEVELOPING PROCESS WHEREIN A REVERSAL DEVELOPMENT IS CARRIED OUT WHILE APPLYING AN EXTERNAL VOLTAGE, THE IMPROVEMENT COMPRISING VARYING SAID EXTERNAL VOLTAGE SO AS TO BE SYNCHRONIZED WITH THE ATTENUATION CHARACTERISTICS OF A LIGHT SENSITIVE LAYER IN CONTACT WITH THE LIQUID DEVELOPING SOLUTION, AND THEREBY APPLYING A VOLTAGE SUFFICIENT TO JUST NEGATIVE AN ELECTRIC FIELD BY ELECTRIC CHARGES OF A MAXIMUM CHARGE DENSITY OR A VOLTAGE SLIGHTLY LESS THAN THAT.

Feb. 2, 1971 SATORU HONJO ETAL 3,560,203

ELECTROPHOTOGRA PHIC DEVELOP 1N6 PROCESS Filed Nov. 2, 1967 INTENSITY OF INTENSITY OF I ELECTRIC 1 ELECTRIC 1 P HELD (E); i 1 l M E=0 b E 0 W b N I l I l 1' I I Z\ a 1 AM? He- 3 f E' 0F INTENSITY EL TRIC (E) FIELD OF ELECTRIC H G .4 E=0 1 I b FIELD 3 m L ummnm H Q 0 T, T2 TIME INTENSITY 0F ELECTRIC FIELD E0 FIGS INVENTORS SATORU HONJO MASAMICHI SATO I I 0 ,4,4..,2,M,M,

o r, T 1mg BY ,.w(

ATTORNEYS United States Patent 3,560,203 ELECTROPHOTOGRAPHIC DEVELOPING PROCESS Satoru Honjo and Masamichi .Sato, Kita-Adachigun, Saitama, Japan, assignors to Fuji Photo Film C0., Ltd.,

Kanagawa, Japan Filed Nov. 2, 1967, Ser. No. 680,227 Claims priority, application Japan, Nov. 2, 1966, 41/ 72,265 Int. Cl. G03g 13/00 US. Cl. 96-1 7 Claims ABSTRACT OF THE DISCLOSURE In an electrophotographic liquid developing process wherein a reversal development is carried out while applying an external voltage, the improvement comprising varying said external voltage so as to be synchronized with the attenuation characteristics of a light sensitive layer in contact with the liquid developing solution, and thereby applying a voltage sufficient to just negative an electric field by electric charges of a maximum charge density or a voltage slightly less than that.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to an electrophotographic developing process and more particularly, it is concerned with a developing process wherein a reversal development is carried out while applying a voltage.

(2) Description of the prior art In the electrophotographic process, an electrostatically detecting powder (which will hereinafter be referred to as toner) is adhered to areas where electrostatic charges are present or the toner is adhered to areas where electrostatic charges are not present, the latter being called a reversal developing process.

The present invention, relating to the reversal developing process, is illustrated by taking a xerographic process as an example. A light sensitive material is used comprising an electrically conductive support and a photoconductive layer, such as an amorphous selenium or a mixture of zinc oxide powder and a resin. The light sensitive layer is uniformly charged by a suitable method, for example, by a corona discharge in a dark place, and then exposed to light imagcwise, whereby the electric charges on the exposed light sensitive layer are neutralized and eliminated, while the electric charges on the other areas remain. Consequently, the electrostatic charges appear as an image on the light sensitive layer according to the light image. When a toner having the same sign as the electric charges on the light sensitive layer is contacted therewith, it adheres to the charge-free areas. The distribution of electric field on the light sensitive layer is represented by electric lines of force in FIG. 1.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be illustrated by the accompanying drawings, in which:

FIG. 1 is an illustrative representation of the distribution of electric field on the light sensitive layer in the electrophotographic process;

FIG. 2 is an illustrative representation of the effect of and adjacent electrode used for preventing the edge effect;

FIG. 3 is an illustrative representation of the application of a voltage to the light sensitive layer of FIG. 2;

FIG. 4 is a graph representing the dark attenuation characteristics of the charged light sensitive layer; and

FIG. 5 is a graph representing the dark attenuation characteristics in a case where the charged light sensitive layer is allowed to stand in air for a short time, and then is immersed in an insulating liquid.

SUMMARY OF THE INVENTION It is the principal object of the invention to provide an electrophotographic developing process wherein, in the reversal developing effected by the use of a liquid developing agent while applying an external voltage, fogging due to the attenuation characteristics of a light sensitive layer in the liquid can be effectually prevented. The process is characterized by varying the external voltage so as to be synchronized with the attenuation characteristics of a light sensitive layer and thereby applying constantly a voltage sutficient to just negative an electric field by electric charges of maximum charge density on the light sensitive layer or a voltage slightly less than that, or applying a constant external voltage calculated by, considering the strength of the electric field attenuating during developing according to the attenuation characteristics of the light sensitive layer, subtracting at least a voltage corresponding to the electric field.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1a illustrates the direction and distribution of electric lines of force in cross section where there are positive charges of a band form on light sensitive material 1 comprising light sensitive layer 11 and electrically conductive support 12. FIG. lb represents a distribution of electric field near the surface of the light sensitive layer in adirection vertical to the light sensitive layer corresponding to FIG. 1a. The horizontal axis corresponds to the position of the light sensitive layer and the vertical axis represents the strength of the electric field. As is evident from FIG. lb, there is formed a positive electric field (a direction vertically outwards from the surface of the light sensitive layer is defined as positive) within a range in which there are positive charges, but the distribution of the electric field is not uniform. That is to say, the strength of the electric field is relatively small inside a band part (which will be hereinafter referred to as band) where there are electric charges, maximum near the ends of the band, and substantially zero at the ends of the band. On the other hand, the electric field is negative outside the band, the strength thereof being maximum near the ends of the band and getting smaller farther from the band. When a positively charged toner is brought into contact with the light sensitive layer. the toner is attracted by negative areas of the electric field, whereby development is carried out in a pattern next to the distribution of the negative electric field of FIG. 1b. Therefore, the development is finished very densely just outside the band, but is thinner as the distance from the band gets larger. This phenomenon has been well known as edge eiTect in the field of electrophotography.

Only areas near the boundary with the band where electric charges are present is developed densely, in spite of the natural requirement that charge-free areas should be uniformly developed. In order to overcome this disadvantage, the process is carried out using an adjacent electrode. FIGS. 2a and b represent the electric lines of force and distribution of electric field in this case. That is, FIG. 2a shows that the electric lines of force are substantially vertical to the light sensitive layer when adiacent electrode 2 is arranged substantially in parallel near light sensitive layer 11, while b gives the distribution state of electric field at this time. There are thus obtained a positive electric field of a substantially uniform strength inside the band and a negative electric field next to zero outside the band. Although, if the space between the adjacent electrode and light sensitive layer can be reduced to a very small size, the distribution of the positive electric field ought to be rectangular inside the band and zero outside the band. However, it is difiicult to reduce this space beyond a certain degree because, in practice, toner must be inserted in the space. Therefore, contact of a positive toner, while keeping some space, results in a little adhesion of the toner outside of the band. In order to adhere a toner effectively to areas outside the band, it is necessary to apply an external electric field as shown in FIG. 3. In FIG. 3a, a voltage Va is applied by power source 3 to neutralize the electric field of positive charges on the light sensitive layer.

Consequently, the electric lines of force directed to the adjacent electrode from the band disappear, whereas the electric lines of force directed to the light sensitive layer from the adjacent electrode appear outside the band. FIG. 3b shows the distribution of electric field in this case which is substantially zero inside the band and a substantially uniform, negative value outside the band. When a positively charged toner is contacted with the light sensitive layer under such conditions, it is adhered to the outside of the band in a substantially uniiform and sufficient density, whereby the object of the reversal developing can be accomplished. In this case, an applied voltage Va may be less than that. Where Va just neutralizes the electric field by positive charges of the band, Va is at its upper limit, and, where Va is less than this, there are formed a positive electric field inside the band and a negative electric field outside the band. This negative electric field may have only the strength required to cause a toner to adhere to the light sensitive layer.

In the foregoing illustration, the dark attenuation of the light sensitive layer is left out of consideration. Since a practical light sensitive layer is followed by dark attenuation, more or less, however, the electric charges on the light sensitive layer decrease with the passage of time and the electric field thereby decreases correspondingly. FIG. 4 is a graph representing the dark attenuation characteristics of the light sensitive layer, in which the horizontal axis is time and the vertical axis is the electric field in the vertical direction near the surface of the light sensitive layer. Where t=0, electrostatic charging of the light sensitive layer is completed, the electric field at the time being E E is an electric field formed by the initial potential. When developing is started where t=T and completed where t=T the electric field is reduced from E=E where t=T to E=E where t=T E =E may be varied from a very large value to a very small value according to the characteristics of the light sensitive layer. In fact, the value of E E varies sometimes from several volts to several thousand volts. If developing is carried out while applying an external voltage Va which just neutralizes the electric field E where t=T or is slightly less than that, the electric field by the external voltage exceeds that by electric charges of the light sensitive layer sometimes in a period of from t=T to t=T and a toner thus adheres to areas where the toner is not adhered initially through the negativing of both the electric fields, resulting in fogging.

It is found that this disadvantage can be overcome by varying the external voltage so as to be synchronized with the dark attenuation of the light sensitive layer, whereby both of the electric fields are just negatived by each other or the external voltage is kept slightly smaller, or by examining previously the attenuation degree of the voltage of the light sensitive layer during developing and decreasing the external voltage by at least the determined degree.

Although the dark attenuation of the light sensitive layer is taken into consideration in the foregoing illustration, the light sensitive layer, in the case of liquid developing, is often attacked by an insulating liquid whereby the voltage is rapidly attenuated. In a case where 4 the light sensitive layer consists of a mixture of zinc oxide powder and a resin, the resin being a silicone resin, for example, the resin absorbs many of organic insulating liquids even though it is not dissolved therein and the attenuation becomes so rapid that development cannot be carried out. An initial potential, e.g., 500 v., becomes sometimes next to zero after about 10 seconds when contacted with kerosene. Although it is possible to reduce such attenuation by changing the variety of resin, many of resins, for example, hardened alkyd resins, are followed by some attenuation of potential through contact with insulating liquids, more than upon standing in the air.

In the case of powder cloud development, magnetic brush development or cascade development, it is necessary only to take a dark attenuation in air into consideration, but, in the case of liquid development, rather the attenuation of potential by a liquid must be taken into consideration in many cases.

FIG. 5 shows the dark attenuation in air and in an insulating liquid, the former being represented by curve A, and the latter by curve B. The light sensitive layer whose surface electric field is B; when i=0 attenuates along the curve A in air, and the attenuation of the surface potential proceeds along curve B with the start of liquid development when t=T When the development is stopped at t=T the surface potential decreases to E This ought to be E at 2:T if in air. It is found often that E E E E In the foregoing illustration, an example is taken where there are electric charges of a constant charge density on certain areas of a light sensitive layer and no electric charge on another area. Since the charge density is continuously distributed from zero to maximum on a light sensitivity according to a light image in fact however, it is to be understood that, in the practical reversal development, areas of maximum charge density may correspond to areas of constant charge density in the foregoing illustration.

What is claimed is:

1. An electrophotographic developing process wherein reversal development is carried out on a photoconductive layer while applying an external voltage, the attenuation characteristics of said photoconductive material being such that the dark attenuation thereof decreases with respect to time during said reversal development, said process comprising the steps of:

uniformly charging said photoconductive layer;

exposing the charged layer to an image to thereby form a latent electrostatic image on said photoconductive layer; applying developing material to said photoconductive layer to thereby adhere said material to the non latent image portions of the photoconductive layer under the influence of said external voltage to effect said reversal development; decreasing said external voltage, while said material is being applied from a first value sufiicient to approximately balance out the electric field resulting from the charges of said electrostatic latent image substantially immediately prior to said application of said developing material to a second value sufficient to approximately balance out the electric field resulting from the charges of said electrostatic latent image substantially immediately after said application of said developing material, said second value being determined by said attenuation characteristic,

whereby said reversal development will be effected with substantially no fogging.

2. A process as in claim 1 where said external voltage is continuously decreased from said first value to said second value in accordance with said attenuation characteristic of said photoconductive layer.

3. A process as in claim 1 where said external voltage is abruptly decreased from said first value to said second value.

4. A process as in claim 1 where said reversal develop- References Cited ment is effected by liquid development and where said UNITED STATES PATENTS second value is also a function of the effect of said liquid on said photoconductive layer. 3,084,043 4/1963 Gunflilach 96 1 5. A process as in claim 1 where said reversal develop- 5 3,383,209 5/1968 Casslers et ment is effected by cascading the development material 3,457,070 7/1969 Watanabe et 96 1-4 over said photoconductive layer.

6. A process as in claim 1 where said reversal develop- GEORGE LESMES Pnmary Exammer ment is eflected by magnetic brush development. WITTENBERG, Assistant EXamiIlel 7. A process as in claim 1 where said reversal develop- 10 US CL X ment is efiected by powder cloud development. 117 37 

